Remove the residual geometry code and datastructures from FFS (keep some kind of
allocation groups but without most of what cylinder groups now have) and replace
blocks and fragments with extents, yielding a much simpler filesystem well
suited for modern disks.

Note that this results in a different on-disk format and will need to be a different file system type.

The result would be potentially useful to other operating systems beyond just
NetBSD, since UFS/FFS is used in so many different kernels.